The 15 threatened animals were shot dead for their horns last month in the Mozambican part of Great Limpopo Transfrontier Park, which also covers South Africa and Zimbabwe.

They were thought to be the last of an estimated 300 that roamed through the special conservation area when it was established as "the world's greatest animal kingdom" in a treaty signed by the three countries' then presidents in 2002.

The latest deaths, and Mozambique's failure to tackle poaching, has prompted threats by South Africa to re-erect fences between their reserves.

Wildlife authorities believe the poachers were able to track the rhinoceroses with the help of game rangers working in the Limpopo National Park, as theMozambicanside of the reserve is known.

A total of 30 rangers are due in court in the coming weeks, charged with collusion in the creatures' deaths, according to the park's administrators.

Conservationists say the poorly-paid rangers were vulnerable to corruption by organised poaching gangs, who target rhinoceroses for their horns which are prized in Asia for their reputed aphrodisiac and cancer-curing properties.

The trade in rhino horn has seen the numbers of rhino killed spiral in recent years. Over the border in Kruger, the South African part of the transfrontier park, 180 have been killed so far this year, out of a national total of 249. Last year, 668 rhino were poached in South Africa, a 50 per cent increase over the previous year.

Kelvin Alie, from the International Fund for Animal Welfare, said the fact that the rangers may have been turned while working on such an important conservation initiative "speaks volumes about the deadly intent of the wildlife trade".

"They will stop at nothing to get to their quarry," he said. "It is tragic beyond tears that we learn game rangers have now become the enemy in the fight to protect rhino from being poached for their horns."

A spokesman for South Africa's environment minister said she would be meeting her Mozambican counterpart in the coming weeks amid concerns that the country is not pulling its weight in the battle against poaching.

"Clearly the open fence agreement has become an open season for poachers," Albi Modise said. "Rangers in the Kruger National Park are engaged in daily battles with Mozambican poachers."

Dr Jo Shaw, from the World Wide Fund for Nature, said the rhinoceroses had probably crossed into Mozambique from Kruger.

Whereas killing a rhino in South Africa can attract stricter punishments than killing a person, in Mozambique offenders generally escape with a fine if they are prosecuted at all.

"Rhinos being killed in Kruger are mostly by Mozambican poachers who then move the horns out through their airports and seaports," she said. "With huge governance and corruption issues in Mozambique, it's a huge challenge."